This invention relates to corrosion resistance in iron-based alloys exposed to gases containing oxygen and/or sulfur and particularly to Fe-Cr and Fe-Cr-Ni alloys exposed to oxygen-sulfur mixed-gas environments.
Iron-based alloys and particularly those alloys with Cr and Cr-Ni may be identified as structural alloys for use in the fabrication of coal gasifiers, coal combustion units, and other equipment designed to operate at temperatures in the order of 500-1000.degree. C. Upon exposure to aggressive gaseous environments at elevated temperatures, these alloys become dependent on their ability to form and maintain protective surface oxide scales of Cr.sub.2 O.sub.3. However, in oxygen-sulfur mixed-gas environments, typical of those encountered in coal gasification and combustion atmospheres, it has been recognized that a thermodynamically stable protective oxide may not form owing to the presence of sulfur in the gas phase. In addition, the presence of sulfur also may significantly increase the corrosion rate on these alloys over that in air or other oxygen atmosphere.
In U.S. Pat. No. 4,470,848 issued Sep. 11, 1984 for Natesan and Baxter and assigned to the U.S. Government, the invention involved the improved high-temperature performance of iron-based alloys to oxidative and/or sulfidative conditions by the addition of about 1-8 wt.% of Nb or Zr to the bulk alloy.
In general, it was believed that the additive would form intermetallics with iron and any nickel with the chromium concentration thereby increasing in the alloy matrix to form Cr.sub.2 O.sub.3 as a protective scale on the surface. In providing the desired protection, it was particularly useful to anneal the alloy after addition of Nb or Zr at a temperature of 1100.degree. C. for up to 7 h. One purpose of this treatment was to aid in the migration of Nb or Zr to the surface portion of the structure to promote the formation of Cr.sub.2 O.sub.3. Subsequently, it was also found that for Nb, a layer of oxidized Nb was also formed at the exposure temperature below the protective Cr.sub.2 O.sub.3 with the internal oxide layer acting as a barrier to migration outwards of iron from the substrate alloy.
While the invention in U.S. Pat. No. 4,470,848 provided improved high-temperature resistance of iron-based alloys to attack by corrosive gases containing oxygen and sulfur, new protective techniques were of interest to avoid the limitations resulting from the high temperature annealing at 1000-1100.degree. C. and the requirement for the addition of the Nb or Zr to the bulk alloy. In particular, it is important to provide protection to these iron-based alloys which are exposed to lower temperatures of 100-700.degree. C. during heat treatment or under process conditions. Under these circumstances, the concentration of Nb or Zr required in the bulk alloy may be considerable in order to provide the desired concentration at the surface region of the alloy to resist attack by oxygen-sulfur mixed gas environments.
Accordingly, one object of this invention is an iron-based alloy with protection against attack by corrosive gases containing oxygen and sulfur. Another object of this invention is an Fe-Cr or Fe-Cr-Ni alloy with resistance to those corrosive gases in which the processing of the alloy does not require annealing at 1000-1100.degree. C. Still another object of the invention is an Fe-Cr or Fe-Cr-Ni alloy with the desired corrosion resistance without the addition of Nb or Zr to the bulk alloy. These and other objects of the invention will become apparent from the following description.